How would I construct my hypothesis for which bubble gum brand blows the largest bubbles.

You might want to identify and name the brands of the bubble gum you want to use for the test.

A hypothesis is just a simple statement of what you want to prove.

I Googled with a few keywords from your question and found entire science experiments for exactly what you want to do.

An hypothesis would be a theory about why a particular composition of gum blows bigger bubbles. If you don't know how the different brands make their gum, and out of exactly what ingredients, and in what proportion, then it's nothing but a test.